P380: EX VIVO IMMUNE ACTIVATION WITH THE MACROPHAGE-TARGETING IMMUNOTHERAPY, ANTI-CLEVER-1 ANTIBODY BEXMARILIMAB, IN ACUTE MYELOID LEUKEMIA AND MYELODYSPLASTIC SYNDROME
Authors: S. Aakko, A. Ylitalo, H. Kuusanmäki, M. Björkman, J. Mandelin, J. Jalkanen, M.-L. Fjällskog, C. Heckman, M. Hollmén, M. Kontro
Journal: HemaSphere (2022)

Background: Despite the recent approvals of targeted therapeutic agents for acute myeloid leukemia (AML), the treatment options remain few and for myelodysplastic syndrome (MDS), even fewer. The potential of immunotherapies in these myeloid malignancies remains under investigation. To this end, bexmarilimab, an anti-Clever-1 antibody, aims to harness the therapeutic potential of macrophages. Clever-1 (STAB1) is a scavenger receptor, expressed on the surface of a subpopulation of immunosuppressive macrophages. By inhibiting Clever-1, bexmarilimab demonstrates potential to turn the anti-inflammatory macrophages to pro-inflammatory and activate CD8+ T cells (Virtakoivu R. et al. 2021. Clin. Cancer Res) along with promising anti-tumor activity against solid tumors in patients with multiple lines of previous treatment (Bono P. et al. 2019. Ann. Oncol.). As Clever-1 is expressed in myeloid cells and high STAB1 levels associate with poor survival in AML (Lin S.Y. et al. 2019. Mol. Ther. Nucleic Acids), bexmarilimab may possess therapeutic potential in AML and MDS. Aims: The aim of this study was to profile Clever-1 expression in AML and MDS and to test in preclinical models bexmarilimab’s growth inhibitory and immunomodulatory potential, as a single agent and in combination with azaciditine and venetoclax. Methods: AML cell lines (n=11) and frozen mononuclear cells, extracted from AML (n=27) and very high risk MDS (n=4) patient bone marrow (BM) aspirates provided by the Finnish Hematology Registry and Biobank, were used in this study. Samples were treated for 48h with bexmarilimab alone, or in combination with azacytidine and/or venetoclax. Flow cytometry was used to detect different myeloid and TBNK cell populations along with Clever-1, HLA-DR, PD-(L)1 and Granzyme B protein expression. Results: Our results confirm Clever-1 protein expression in AML cell lines with high STAB1 level and most importantly, in AML and MDS patient-derived BM monocytes and blasts. AML samples from FAB M4/M5 subtypes exhibited highest Clever-1 levels, along with a FAB M2 sample collected prior to allogeneic stem cell transplantation and consequent rapid relapse. Clever-1 expression correlated negatively with monocyte MHC class II molecule, HLA-DR, expression, and BM T cell frequency, in line with the immunosuppressed state associated with high Clever-1. Ex vivo treatment of the primary AML cells with bexmarilimab resulted in a notable, 5-10x fold increase of monocyte HLA-DR in samples with low basal HLA-DR and high Clever-1. The combination of azacitidine with bexmarilimab augmented HLA-DR induction by 20-110% (mean 44%). On the other hand, combination with bexmarilimab overcame the HLA-DR suppressing effect of venetoclax. The BM TBNK cell populations and thus bexmarilimab’s effect on the effector cells, showed great inter-sample variability. FAB M1/M2 AML showed increase of activation markers, such as Ki67, CXCR3 and Granzyme B after ex vivo bexmarilimab treatment in CD8+ T cells. Furthermore, bexmarilimab reduced PD-1 expression in NK- and CD8+CXCR3+ T cell populations of FAB M0-M2 AML and MDS-EB2. Summary/Conclusion: Our results confirm that Clever-1 is expressed in AML and MDS patient BM blasts and monocytes. Ex vivo treatment with bexmarilimab, alone or in combination with azacytidine or venetoclax, demonstrated increased HLA-DR expression, indicating enhanced antigen presentation capability. Furthermore, NK and CD8+ T cells showed decreased PD-1 and increase of activation markers. These results confirm the therapeutic potential of bexmarilimab in myeloid malignancies and await further validation in clinical trials.